A screen printing machine has been used for printing predetermined characters and/or designs onto a printed article such as a sheet, a plate and a box-like member composed of paper, synthetic resin, ceramic, glass or metal. Such a screen printing machine comprises a screen disposed on the printed article, a printing member such as a skid slid along the screen in a pressurized manner during printing and then disengaged from the screen, and a scraper moved along the screen together with the skid to scrape ink. The scraper serves to coat the screen with ink from the end of the screen where the printing begins, while the skid slides on the screen in a pressurized manner from the end of the screen the printing begins, to the end of the screen where the printing terminates to print the article disposed under the screen.
It should be noted that the skid is required to forcibly engage the whole face of the screen in a uniform manner in a widthwise direction of the screen during printing so that an ink film having a uniform thickness is coated over the entire surface of the article to be printed.
The prior art machine comprises means to support the skid as shown in FIG. 5 to meet the aforementioned requirement. A screen 1 is forced onto a surface of an article 2 to be printed such as a sheet-like member and skid 3 disposed on the screen 1 in a liftable manner forcibly engages the screen 1 during printing. The skid 3 is supported in a liftable manner by and below a moveable plate 5 which is in turn slidably supported through sliders 5a, 5a by a pair of shafts 4, 4 provided above the screen 1 and extending in the printing direction. More particularly, a liftable plate 6 of a lifting block having the skid 3 provided thereon is liftably supported on the moveable plate 5 through a pair of guiding bearings 7a (only one of which is shown in FIG. 5). The liftable plate 6 is connected through a metal joint 8b to a piston rod 8a of an air cylinder 8 which constitutes means to liftably drive the liftable plate 6. The air cylinder 8 is mounted on the movable plate 5. Thus, it will be noted that the liftable plate 6 can be lifted and lowered by the air cylinder 8. A slider mounting holder 6a is provided on the liftable plate 6 and the skid 3 is connected to and supported by the slider mounting holder 6a through a supporting pin 3a so as to be swingable. A pair of adjusting screws 9, 9 threaded through the movable plate 5 are provided on both sides of the slider mounter holder 6a and engage the right and left ends of the skid 3 to swing the skid about the supporting pin 3a.
The pair of adjusting screws 9, 9 are alternately adjusted so that the sliding edge 3b of the skid 3 engages the surface of the article 2 to be printed on a table (not shown) in a horizontal manner. It should be noted that this adjustment has to be made so that the right and left sides of the skid are balanced while the gap between the sliding edge 3b of the skid and the table is observed or while the gap is measured by a pulling force detected by a gap gauge interposed therebetween.
However, it will be understood that the operation of adjusting the skid 3 so that the sliding edge 3b is oriented parallel to the table (the surface of the acticle to be printed) is very troublesome, which means the operation requires a high skilled operator and a large amount of time. This required time for setting the skid 3 contributes to a low efficiency in the operation of the screen printing machine. Furthermore, it is often difficult to obtain a complete parallelism of the skid even though the adjustment is made. Also, even though the skid 3 is kept parallel to the surface of the article to be printed, the skid is locked to the slider mounting holder 6a after the adjustment thereof. Thus, it will be noted that a uniform forcible engagement of the skid 3 with the screen cannot be obtained if a variation in the thickness of the article 2 occurs or curvature thereto develops. Thus, a uniformly forcible engagement of the skid with the screen cannot be obtained and an ink film having a uniform thickness cannot be provided on the surface of the article to be printed.
Furthermore, if the printing is carried out while the skid is not maintained parallel to the surface of the article to be printed, only one of the sides of the sliding edge 3b engages the screen 1 or one side of the sliding edge 3b forcibly engages the screen under a high printing pressure that is beyond the required value thereof, which disadvantageously reduces the printing accuracy.
Even in a screen printing machine in which a lifting block having the skid 3 mounted thereon is swingingly supported on the movable plate 5 so that the skid 3 can follow the inclination of the surface of the article to be printed, when only one side of the sliding edge 3b engages the screen 1 friction will be generated so that the skid 3 will not be able to slide laterally or so that the skid 3 will slide laterally with some time lag after the skid 3 begins to slide. Thus, it will be noted that even such a screen printing machine cannot print the article in a uniform manner and tends to produce the lateral sliding movement of the screen 1 when skid 3 slides laterally.
It will be understood that similar problems will also occur during the lowering operation of the scraper (not shown) which is supported on the movable plate 5 beside the skid 3 and is raised and lowered with the skid 3.